The sophistication of communications systems involving subscription television is continually increasing. The systems being installed at this time have greatly improved communications flexibility as compared to those previously in use, while the quality of the transmitted signal is continually being improved, as is flexibility in the business arrangements possible between the subscriber and the broadcaster. For example, in U.S. Pat. No. 4,484,217 to Block et al., there is disclosed a subscription television system in which all billing calculations are carried out at the decoder, that is, at the individual subscriber's location. When a subscriber makes a payment, e.g., by simply mailing a check to the broadcaster, the broadcaster then sends a message which is detected by that particular decoder and used by it to update the user's credit and/or account balance information. The decoder then controls whether the user is to have access to a particular program or not in accordance with this billing information, possibly also in accordance with other sorts of access controls transmitted by the broadcaster. For example, each program transmitted may include an indication that it belongs to a particular "tier" or class of programs. The decoder then permits access to the program if programs of that tier had previously been authorized.
The Block et al. patent also discloses that teletext data, that is, textual information relating to stock market reports and the like, could also be transmitted using this system. However, the teletext data discussed in the Block et al. patent appears to be system-wide data, that is data relevant to all subscribers of the system more or less equally. The Block et al. patent does not teach any way in which, for example, an individual broadcaster may send a chosen subscriber a personal message other than billing update data, as above.
"Specification of Standards for Broadcast Teletext Signals," a document published by the British Broadcasting Corporation et al. in September, 1976, describes the teletext broadcasting system in effect in the United Kingdom. According to this system, teletext information such as closed-captioned programming is available for the hearing-impaired. According to this system, as understood by the applicants, the subscriber simply depresses a button on a key pad comprised by the decoder, and the decoder then selects alphanumeric data which is transmitted during the vertical blanking interval of the standard television signal. The decoder can then add this closed-captioned textual material directly to the video screen, so that it can be seen by the hearing-impaired. Again, however, this material is transmitted system-wide, and there is no provision made for transmission of individual messages to individual subscribers.
Those skilled in the art will recognize that there are a number of constraints on the operation of any successful subscription television system. For example, there must be an extremely reliable and trustworthy method of computing charges to be billed to the subscriber and for communication of credit status updates. It is also desirable that the subscriber be permitted to prepay for certain classes of service, for example, to have access to a particular broadcaster's offerings. On the other hand, it is also desirable that an "impulse" purchase system be provided such that the viewer could purchase a particular program on a "pay-per-view" basis when desired. Similarly, it is desired that a running list of programs purchased by the subscriber be available, such that he can confirm that he is indeed being billed correctly. It is furthermore desired that means be provided for securing the system against improper use, which includes both prevention of viewing of inappropriate subject matter and purchase of fee-bearing material by persons not authorized to do so.
The requirement that the credit status be updateable further requires that communication be possible in both directions between the decoder and the transmitter. However, it would be inefficient to provide individual uplink transmission facilities at each decoder, for example, in a satellite TV system. Therefore, it is desirable that a two-way communications facility be provided connecting each decoder with the transmitter, but in which the two data links are not made using the same medium. Therefore, the communication facility which carries subscription television signals from the transmitter to the decoder is not the same as that used to carry payment status information, queries or the like from the decoder to the transmitter.
As indicated above, the prior art suggests that individual decoders can be addressed for transmission of billing information. It would also be desirable if individual user messages could be transmitted to individual users such as to provide an additional communications facility not previously made available. If this is to be done, there are several criteria which must be met. Paramount among these is security; preservation of the secrecy of the message and its availability only to the particular addressee is of utmost importance. At the same time, it is desired to be able to address messages to classes of subscribers, for example, to transmit civic data or the like.
Another complexity in providing a secure subscription television system is the fact that the decoder cannot be considered to be in "friendly hands." That is, unlike an encrypted data channel as used by financial institutions or in military applications, in which both transmitter and receiver are in friendly hands, an additional security exposure which must be faced in connection with a television system is analysis of the transmitted signal. In a subscription television system, the transmitted signal must, of course, be protected, as a very important threat is posed by viewers who have purchased a decoder and attempted to modify it to bypass the security mechanisms, the billing arrangements or the like. A further problem is posed by those who would make copies of, for example, television movies using videotape recorders or the like for subsequent viewing or resale.
It is, therefore, desirable that a system be provided which provides a very high level of security, both as to the transmitted signal and as to the operation of the decoder, and one in which some sort of "fingerprint" identifying the source of an unauthorized copy can be provided.